This invention relates to an image signal processing device used in image signal reproduction devices such as laser vision disc players and video tape recorders and, more particularly, to an image signal processing device capable of preventing occurrence of an erroneous operation caused by insertion of a copy guard signal in an image signal reproduced by such image reproduction devices.
There are laser vision disk players and video tape recorders which have a function of displaying various characters such as channel numbers, volume levels, chapter numbers, frame numbers and operation modes superimposed on an image displayed on a television screen. Positions of these superimposed characters to be displayed are controlled normally by counting the number of horizontal synchronizing signals after occurrence of a vertical synchronizing signal.
Some recorded original video tapes and discs have a so-called copy guard signal recorded thereon for preventing copying thereof. Among various methods for prohibiting copying of a video tape or disk by using a copy guard signal, there is generally known a method according to which a synchronizing signal is modified so as to prohibit synchronization when a copied tape or disk is reproduced and thereby making a reproduced picture of such copied tape or disk too instable to watch properly. In this type of copy guard method, the copy guard signal is recorded in a section following an equalizing period in a vertical retrace blanking period as shown in FIG. 2A.
A horizontal synchronizing signal is detected generally by comparing an image signal with a predetermined threshold value. When an image signal including a copy guard signal as shown in FIG. 2A is compared with a threshold value Vth, the result of comparison becomes an output shown in FIG. 2B which includes the copy guard signal.
For this reason, when the display position of characters to be superimposed on an image is determined by counting the detected horizontal synchronizing signal, superimposed characters such as a channel display 12 and volume display 14 on a television screen 10 shown in FIG. 3 are shifted from correct positions shown by solid lines to incorrect positions shown by dotted lines.
It is, therefore, an object of the invention to provide an image signal processing device capable of preventing occurrence of an erroneous operation caused by insertion of a copy guard signal in an image signal in a signal processing operation such as production of a superimposed character.